The invention relates to a powertrain equipped with an optimized energy recovery system comprising a flywheel.
The reduction of the fuel consumption is a major stake for the sustainability of many industries, but most important for the automotive industry and the machinery industry. A huge majority of vehicles (trucks, buses, passenger cars, etc.) is fitted with a powertrain comprising an internal combustion engine which drives the driving wheels through a transmission set (including for example a clutch or a torque converter, a manual, automated or automatic gearbox, a differential, an axle) and on another part the auxiliaries that are necessary to operate the vehicle systems. Some of these auxiliaries are fed by an electrical network, which energy comes from a generator run by the ICE. In the case of commercial vehicles, the engine is most often a turbo-charged diesel engine. Similar powertrains are used to power all sorts of machines, including construction equipment machines.
When analyzing the balance of energy used to operate a vehicle, there is a non reducible energy demand due to the drag and rolling forces that are intrinsic characteristics of the vehicle. This one being put apart, an amount of the energy which is used to drive the vehicle is wasted in the brakes, coming from potential energy kinetic energy (resulting from the energy provided to the vehicle when accelerating), and potential energy (resulting from energy provided to the vehicle during hill climbing).
Some well known technologies can limit a fraction of these losses. For example, hybrid drivelines are a known technology to achieve braking energy recovery.
A first of these technologies comprises hybrid powertrains which equip the now well-known Hybrid Electric Vehicles (HEVs) with electro-chemical storages. On a general level, HEVs comprise a powertrain associating an ICE with at least one electric machine and with at least one storage device (batteries, super capacities, inertia wheels . . . ). Such a system can store an amount of the braking energy in the storage device by using the electric machine as a generator and then, at an appropriate time, redirect this energy to the driveline using the electric machine as a motor to participate to the propulsion of the vehicle.
One interesting layout for a hybrid electric vehicle is the so-called parallel hybrid layout where the transmission set of the vehicle is mechanically coupled (directly or through gearings, belts) in parallel to both the ICE crankshaft and to the electrical machine. During decelerations, the electric machine is used as a generator to slow down the vehicle and the electricity that is produced is stored in batteries or in ultra-capacitors. During accelerations, the electric machine is used as a motor and adds its power to that of the ICE, or even replaces the power of the ICE.
Hybrid powertrains are also known in the field of construction equipment machinery.
As a variant, it is also know to have hybrid powertrains with electro-mechanical storage means. Instead or in addition of batteries, an electro-mechanical device can be used where the energy is stored in kinetic form, for example in a spinning wheel also called a flywheel. In this context, the flywheel is a dedicated energy accumulating flywheel which is not to be confused with the ICE flywheel, the sole purpose of which is to smoothen the rotation of the engine. An energy accumulating flywheel needs to store a significant amount of energy which can be enough to drive the vehicle, at least as a complement to the ICE.
In a known layout, the flywheel is mechanically linked to a first electric machine, the purpose of which is to speed up or slow down the flywheel in order to increase or decrease its kinetic energy and transform it into electricity. In a parallel HEV, the electric energy derived from the flywheel can then be used in a second electrical machine to drive the vehicle. The second electrical machine is also used as a generator, during deceleration, to provide the first machine with the electricity to drive the flywheel.
Compared to electrochemical batteries, flywheels are likely to be an economic alternative when:                rapid discharge and recharge are necessary;        long life cycle is necessary;        weight or volume is constrained;        use of toxic or explosive materials is unfeasible; and/or        environmental control is difficult.        
Compared to ultracapacitors or batteries, flywheels are likely to be the economic alternative when industrial voltages must be supported.
When high energy capacities are needed, flywheels must rotate at high speeds with safety issues (risk of burst of the flywheel due to inertia forces) and technological issues (such as issues with the bearing). In the above described layout of a hybrid powertrain equipped with a flywheel, the two electrical machines need to be rated to the maximal power to be recovered. Moreover, in such a layout, all the energy recovered by the system has to be first transformed from mechanical form to electrical form, then from electrical form to mechanical form to be stored in the flywheel, and again from mechanical form to electrical form, then from electrical form to mechanical form to be used as driving power. Although electric machines have inherently good efficiency ratios, these numerous transformations necessarily lead to energy losses.
Document US-2007/0049443 discloses another type of layout for a vehicle equipped with an energy recovery system. The layout is based on a conventional ICE driveline with an ICE, a torque converter with a lock clutch, an automatic gearbox and a transmission shaft which is coupled to the final drive. On this base, an energy accumulating flywheel is coupled to the output shaft of the gearbox through a three-way power split transmission device comprising three input/output couplings, a first of which is coupled to the flywheel, a second of which is coupled to a first electrical machine and the third of which is coupled to the transmission shaft of the gearbox. The three way power split transmission is embodied as a planetary gear. The system further comprises a second electrical machine which is directly coupled with the engine output shaft. Essentially, the first electrical machine and the planetary gear form a continuously variable transmission between the flywheel and the transmission shaft, said transmission being electrically controlled by the first electrical machine.
Compared to the above described conventional layout, the layout of document US-2007/0049443 is more favorable because part of the energy recovered through the flywheel is transferred purely mechanically, that is with a high efficiency ratio. Nevertheless, this layout has some drawbacks. As can be seen from the graph of FIG. 2 in document US-2007/0049443, during acceleration or deceleration of the vehicle, the speed of the flywheel (which corresponds to its “state of charge”) is heavily dependant on the vehicle speed. To keep the flywheel at a certain speed, or to bring it to that speed, the first electrical machine is constantly operated within an extended range of speeds. Only for one defined vehicle speed is the first electrical machine at rest, at which moment the recovered energy is transferred entirely mechanically. For whatever other speed, part of the energy is transferred electrically, and the more the vehicle speed is different from the defined speed, the more important the electrical part. This layout, with the three way power split transmission on the transmission output shaft, also leads to relatively big torque values being transferred trough the recovery system, which therefore need be dimensioned consequently.
Therefore, it is desirable to provide a new layout for powertrain equipped with an optimized energy recovery system comprising a flywheel, so that the size of the system can be reduced, so that it can be better integrated on a vehicle or a machine, and so that it can have a better efficiency ratio.
An aspect of the present invention provides for a powertrain comprising:                a variable ratio transmission device having an input shaft coupled to an engine and an output shaft coupled to a driven unit        a three-way power split transmission device comprising three input/output couplings, a first of which is coupled to a flywheel and a second of which is coupled to an electrical machine, characterized in that the third input/output coupling of the three way power split transmission device is mechanically coupled to the engine and to the input shaft of the variable ratio transmission device.        